Re: [NTLK] External Power Packs for Newtons

From: Johannes Wolf (mp2100_at_mail-gw.estec.esa.int)
Date: Mon Apr 04 2005 - 08:25:57 PDT


Sonny,

the answer to your 1st question is - YES! If you go for alkalines for
instance, you can just use 5 cells in series. But I wanted to make a more
general statement: using a dc/dc converter you could use ANY power source in
within the allowed input voltage range of the converter.

But this is strongly linked to my second point - preventing the Newt from
recharging its internal battery pack.

Why? Imagine you are in a desert without any power source. What you really
want is simply to keep your Newt running as long as possible. This is
definitely not achieved when recharging the internal battery pack first
because of the additional losses.
You would be much better off using a, lets say baby size, alakaline power
pack with 5 cells.

But again - NEVER EVER do that with a rechargeable battery pack inside the
Newt!!!!!
No charging - no problem at all with limiting the current.
If you do not prevent the Newt from recharging its battery pack YOU HAVE TO
LIMIT the current into the Newt according to the ac adaptor specification,
otherwise you DEFINITELY BLOW your Newt.

But I do not really see the point using an external power pack with
2700mAh - you can have that easily inside the Newt - in a battery tray.

And using a higher input voltage with a dc/dc converter would lower the
current on the input side. Thus an external pack with alkalines would last
really long.
You can do an easy calculation:
lets start with the nominal current that a Newt is drawing without charging
the batteries, that is in the range of 50 - 70 mAmps.
Lets further assume that the converter provides an efficiency of 80%.
If cou consider an input voltage of lets say 9V (6 x 1.5V in series), then
we have the following power budget:

7.5V x 0.05A = 0.375W
plus 20% losses due to the converter (rather conservative) = 0.450W
at 9V this would mean a current of 0.450W / 9V = 0,05A
at 12V this would mean 0.0375A
To estimate how long the Newt would be operable you take the figure of the
current at the primary side we just calculated and divide the given capacity
of the cells by that value.

Cheers
Johannes

> -----Original Message-----
> From: newtontalk-bounce_at_newtontalk.net
> [mailto:newtontalk-bounce_at_newtontalk.net]On Behalf Of Sonny Hung
> Sent: Monday, April 04, 2005 4:37 PM
> To: newtontalk_at_newtontalk.net
> Subject: Re: [NTLK] External Power Packs for Newtons

> On Apr 4, 2005 5:15 AM, Johannes Wolf
> <mp2100_at_mail-gw.estec.esa.int> wrote:
> > The most important things are:
> > 1) to stabilise te output voltage to 7.5V max.

> Wouldn't this mean just 5 Battery Cells to keep the limit?

> > 2) to prevent the Newt from charging, in this case the current
> limitation
> > required for the ac power adaptor is not important and not
> really needed.

> Tell me why this would be a concern? Being that I'm thinking in
> regards of using a battery power pack? I've had this idea of using a
> commercially available rechargeable external pack that can accommodate
> the Newton. It can be charged with a A/C Adapter or a Car Adapter.
> I've checked the specs out and they are 2700mAh NiMH capacity. Testing
> has proved that it will work fine. However I cam contemplating
> building my own now...

> These are the prices I've seen on one sellers site for a MP2x00
> Re-Celled Rechargeable Pack... now my solution I'm working on is
> higher capacity and still less than the 2200mAh offering for $79.
> It's an External Pack though...

> 2000 mah* (67% longer life) $49:
> 2200 mah* (83% longer life) $79:
> 2500 mah* (over 2X longer life) $109:

> Can you explain why do we need to prevent the Newton from recharging
> the internal NiMH Pack with the External Pack? It would be nice to
> understand this in lay-man's terms.

> Sonny Hung
> the Hung Family

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